Originally posted by PATCHES JUNIOR
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NO ONE has asked for any money in relation to this detector .. and the post says nothing about ground balance ... nothing about gold detection ... nothing about minelab and the 1000 gain you are talking about is completely out of context ... SO CHECK YOUR FACTS OR YOUR EYEGLASSES.
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( copy of my original post below )
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It seems that some are unware of patented active damping techniques or maybe cant read or need a simple explanation.
HOW CURRENT PI detetectors work. ( simplified )
The pulse induction detector as known in the art ( ie the ones you can buy today ) generally has three states.
A. A transmit period during which a current pulse is passed through a coil. eg 200 microseconds
B. A flyback period during which a high voltage back EMF develops across the coil and the peak current in the coil rapidly decreases. eg 5 microseconds.
C. A decay period during which the residual current/voltage from the flyback period decays exponentially. A damping resistor is usually connected across the coil so that the coil does not ring during the decay period. eg up to 100s of microseconds.
The increase and decrease of current in the coil ( di / dt ) during period A and B cause eddy currents to be generated in conductive targets. These eddy currents in most targets will persist beyond period A and B above and can be detected by sampling the decay voltage or current of the coil in period C.
As you swing across a target the decay curve in C will change slightly and this is sampled and amplified and makes a beep or whatever in your headphones. Of course in a well designed detector there will be more carefully timed samples and processing to eliminate ground and the earths magnetic field both of which will appear as false target indications if you dont compensate for them. There are lots of patents and designs focusing on how to sample and pulse timings to improve these areas.
However dear reader there are very few patents on damping ... ( much to my relief ) ... because everyone has assumed that a damping resistor is the optimal way to damp a coil. Damping mathematically or practically applies to a system where you wish to modify an oscillating system. Oscillators are narrowband systems ... when in fact the PI is a broadband pulse system and when we talk about pulse systems we are talking about equalisation and impedance matching ... not oscillators.
Anyway I digress ... back to why the moodz detector is better..... there are a number of reasons but one of them is this.
Lets assume energy stored in the magnetic field during period A ( the tx period ) = 1.0 joule.
During period B ( the flyback ) most of this energy returns to the coil as the magnetic field collapses. Lets say 0.9 joule.
So where has the 0.1 joule gone ? Some is lost or stored in the ground, some is radiated as EMI and some is "stored" in the target as eddy currents.
The decaying eddy currents in the target are detected during period C by your standard PI detector. ( remember the decay curve ).
HOWEVER ... there is a complicating factor .... if a target is a distance X from the coil the eddy currents stimulated in the target during period A and B will be reduced by a factor of 1/XxXxX or one over X cubed. Call this the transmit path loss. During period C the same rule applies there will be a further path loss of 1/XxXxX as the standard PI detector tries to detect a target eddy current during period C. In other words the TX loss multiplied by the RX loss is 1/XxXxXxXxXxX or one over X to sixth power. All of this because the standard detector measures the target "return" signal during period C. This maths has been simplified the real maths still demonstrates the advantage.
The moodz detector does not measure the target return during period C ... there is very little decay curve to sample in period C because the active damping control loop of the moodz detector minimises this "error" voltage. The moodz detector effectively estimates the energy stored in the target ( ie the eddy current ) during transmit by subtracting the energy returned during flyback ( period B ) from the energy transmitted ( period A ) ie instead of measuring C we measure B - A This is just like alegbra :-) ..... except here is the killer ...... by measuring B - A instead of C the path loss to the target is only 1 / XxXxX for the moodz detector instead of 1 / XxXxXxXxXxX for your standard PI detectors. Thusly the moodz detector is 3 orders of magnitude more sensitive than your BrandX detector.
In numbers ... simplified and no ground / circuit or radiation losses .... and target is a distance of 10 units from the coil.
BrandX Detector
Period A : TX Energy = 1.0
Period B : Eddy Current Energy in target = 1/(10 x 10 x 10) = .001
Period C : Target decay RX = .001/(10 x 10 x 10) = .000001
ie RX signal = .000001
Moodz detector
Period A : TX Energy = 1.0
Period B : Return Energy during flyback = 0.999
ie RX signal = A - B = .001
moodz detector is 1000 times more sensitive.
OBVIOUSLY ... because the moodz detector does not try to detect target energy per se during period C ... it dispenses with all that crazy patented sampling schemes used over the years. Target holes ... what are they ? LOL.
There are at least 3 more innovative benefits of the moodz detector arguably as good as the target gain feature which I wont go into here however if you cannot see the benefit of having a 60 dbv increase in target sensitivity then just go and buy the BrandX detectors ... you dont need a moodz detector ... you cant buy one at the moment anyway. :-)
moodz
...he he ... some of this text sounds like it came from a patent
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